1. Field of the Invention
This invention relates to devices and methods for all-optical signal regeneration in optical communication systems.
2. Description of the Related Art
The telecommunication industry has experienced large growth in the last few years and the need for bandwidth is expected to increase further as new Internet-based services are being implemented. Optical point-to-point wavelength-division-multiplexed (WDM) network links were able to fulfill the capacity requirements in the past and have been implemented world-wide. To meet future demand, a next step in optical network evolution will be to implement the routing and switching in the optical domain. A major concern, however, is the accumulation of noise which severely limits the cascadability of optical network nodes.
It is known to regenerate optical signals using opto-electronic regenerators. Such opto-electronic generators are based on a receiver, electronic regeneration, and an optical transmitter.
In addition, different techniques for all-optical 2R regeneration (2R: providing reamplification and reshaping) have been proposed. The main mechanism of such all-optical regenerators is based on a nonlinear power transfer function. A first category of all-optical regenerators provides regeneration both at the upper power level (logical one) and at the lower power level (logical zero) of the optical signal pulses. The regeneration mechanism can be based on a single power transfer function, wherein a same power transfer function is used for the upper power level (logical one) and for the lower power level (logical zero). For example, interferometric regenerators are based on a single power transfer function. Alternatively, the regeneration mechanism can be based on using different power transfer functions for the upper power level and for the lower power level. Such all-optical regenerators can provide a bit error rate (BER) reduction. An example of this type of all-optical regenerators is the so-called Mamyshev-type regenerator. A second category of all-optical regenerators provides regeneration only at the upper power level or only at the lower power level of the optical signal. Such regenerators can be based on devices having a saturation characteristic. For example, semiconductor optical amplifiers can be used to provide regeneration at the upper power level (logical one) of an optical signal.
Mamyshev type all-optical regenerators are based on self-phase modulation of an optical data signal in a nonlinear medium such as a non-linear fiber, with subsequent optical filtering. This type of optical regenerators results in a very good regeneration, but requires a very high input power (e.g. average input power higher than 100 mW) and only works for short pulses. They are generally rather bulky and require long fiber lengths. It has been shown (M. Rochette et al., “2R Optical Regeneration: An All-Optical Solution for BER Improvement”, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 12, No. 4, 2006) that the BER can be improved using this type of regenerator.
An example of an interferometric all-optical regenerator is described in U.S. Pat. No. 6,832,053. Usually this type of all-optical regenerator works with much lower power levels (as compared to Mamyshev type regenerators), e.g. with input power levels in the 1 mW to 2 mW range. However, the quality of the regeneration is worse (e.g. only noise redistribution but no actual BER improvement) and their structure, fabrication, and operation is rather complicated.
In U.S. Pat. No. 6,608,854 a device and a method for all-optical waveform reshaping of optical signals is described, based on gain saturation of an optical amplifier. The device for waveform reshaping includes a distributed feedback (DFB) laser and a drive circuit for supplying a drive current to the DFB laser such that the DFB laser oscillates at a first wavelength included in the stop band. Signal light having a second wavelength not included in the stop band is input into the DFB laser. When the signal light comprises optical pulses each having an upper power level and a lower power level, amplitude fluctuations at the upper level can be suppressed by suitably setting the power of the signal light. Waveform shaping of the optical signal is based on gain saturation of the DFB laser, acting as an optical amplifier. It is shown that amplitude fluctuations at the lower power level can be suppressed by providing an additional component, e.g. a saturable absorber. The input power level needed for this type of optical regenerators can be low, but the regeneration only provides noise redistribution and no BER improvement.